Steering stability and performance of a vehicle are largely characterized by the vehicle's understeer and oversteer behavior. The vehicle is in an understeer condition if the vehicle yaw is less than the operator steering input, where turning the steering wheel more does not correct the understeer condition because the wheels are saturated. The vehicle is in an oversteer condition if the vehicle yaw is greater than the operator steering input. Surfaces such as wet or uneven pavement, ice, snow or gravel also present vehicle stability and handling challenges to the driver. Similarly, in a panic or emergency situation, such as during obstacle avoidance, a driver may react by applying too much steering or failing to counter-steer to bring the vehicle back to its intended path. In any of these cases, the actual vehicle steering path deviates from the intended steering path.
Modern vehicles sometimes incorporate active vehicle control sub-systems that enhance operator comfort and safety, including sub-systems which address such deviations in the vehicle path. One such subsystem is known as a vehicle stability enhancement (VSE) system that assists the vehicle operator in providing vehicle handling. The VSE system helps the vehicle operator maintain control during rapid or emergency steering and braking maneuvers and can correct for understeer and oversteer conditions. The VSE system senses wheel speed, steering angle, vehicle speed and yaw rate. The VSE system uses these inputs to reduce engine torque and apply vehicle braking to maintain the vehicle travel along the intended path.
Another active vehicle control sub-system is known as an active front steering (AFS) system for providing automatic front-wheel steering. AFS systems employ a steering actuator system that receives an operator intended steering signal from a hand wheel sensor, a vehicle speed signal and a vehicle yaw rate signal, and provides a correction to the operator steering signal to cause the vehicle to more closely follow the vehicle operator's intended steering path to increase vehicle stability and handling. The AFS system is able to provide steering corrections much quicker than the vehicle operator's reaction time, so that the amount of operator steering is reduced. In such applications, the AFS system includes yaw rate measurements and feedback control to generate an additional steering input to the front wheels.
Semi-active suspension systems are also incorporated into some modern vehicles and are generally characterized by dampers which are controlled to change the suspension characteristics of the vehicle based on road conditions, vehicle speed, yaw and other considerations. Variable fluid-based dampers are known having discrete damping states and continuously variable damping states. Variability in damping may be attained by variable orifice devices or controlled viscosity fluids (e.g. magnetorheological (MR) or electrorheological (ER)) within the damping device. Variable dampers are used predominantly to achieve low speed ride comfort and high speed handling enhancement. However, variable damping techniques are known to enhance vehicle stability in certain understeer and oversteer situations.
The VSE, AFS and suspension control systems are generally effective at maintaining vehicle stability in light of slowly varying or static road conditions. However, severe and rapidly transient road conditions (e.g. pot holes) effect inputs which may significantly disrupt stability controls when active.
Therefore, it is desirable to account for transient road conditions in vehicle stability systems and minimize the undesirable effects thereof on such systems and controls so that the systems can provide the intended vehicle path across a variety of slowly and rapidly changing road conditions.